Single Top Production Search at CDF

Frontiers in Contemporary Physics III

May 23-28 Vanderbilt University, Tennessee

CDFJulien Donini

University of Padova and INFN

On behalf of the CDF collaboration

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Search for Single Top

Top has been discovered at Tevatron in tt pairs

Single top production predicted by theory: Wtb coupling

s-channelTev = 0.88 ± 0.11 pb

t-channelTev = 1.98 ± 0.25 pb

Wt-associatedTev< 0.1 pb

negligible at Tevatron

Z. Sullivan hep-ph/0408049, T. Tait hep-ph/9909352

Observation of single top: cross section is about 40% of ttbar more difficult to observe due to large background

- important W+jets background

(for Mtop = 175 GeV)

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Single Top Physics

Observation of single top allows direct access to Vtb CKM matrix element cross section ~ |Vtb|2

study top-polarization and EWK top interactionProbe b-quark PDF (t-channel)Test non-SM phenomena

heavy W’ boson anomalous Wtb couplings FCNC couplings like t Z/ c 4th generation

Potentially useful for Higgs searches single top has same final state as Higgs+W (associated) production

Why look for single top ?

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Run I Results

Tevatron Run I results at s = 1.8 TeVTheoretical cross-sections were 30% smaller

t = 1.40 pb s = 0.76 pb

Single top has not been observed: both CDF and DØ set 95% limits t-channel: 22 pb (DØ) 13 pb (CDF) s-channel: 17 pb (DØ) 18 pb (CDF) combined: 14 pb (CDF)

Run II higher beam energy (1.96 TeV) higher rate more luminosity: CDF/DØ have more than 800 pb-1 on tape better detector acceptance

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Signal Topology

General strategy: Separated search of signal of s and t-channels single top production in 1-tag and 2-tag samples Combined search of s+t signal

Event selection: we look for W+2 jets channel one high pT isolated lepton (from W): Et > 20 GeV, || < 1 veto Z, dilepton, convertion events missing Et ( from W): MET > 20 GeV exactly two jets w/ Et > 15 GeV, || < 2.8 ask for at least 1 b-tag

Note: s and t-channel both produce 2 b-jets but bbar from t-channel usually lost into beam pipe

Additional cuts cut on mt mass window: 140 < Mlb < 210 GeV for 1-tag sample, leading jet Et > 30 GeV

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Invariant Mass Mlb Calculation

b choosing in 1-tag events, choose b-tag jet as b from top in 2-tags evt, choose tight jet with larger Q. as the b from top

Recipe is 97% successful for t-channel 53% (75%) successful for s-channel 1-tag sample (2-tags)

Choosing correct neutrino solution Et() = MET pz () is obtained from the constraint Ml = MW:

Two solutions: pick the one with lower |pz ()|

i.e most central solution.Successful matching rate: 74% t-channel, 71% s-channel

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Three methods are currently used and well tested in high pt physics

Soft lepton tagging Use semi-leptonic decay of b-quarks: b lc (BR ~ 20%), b c ls (BR ~ 20%). Leptons with softer pt spectrum than W/Z, less isolated.

Secondary vertex identification Iterative fit on tracks with significant impact parameter Efficiency is 45-50% for central top b-jets Mistag rates are kept typically at 4-5%

Jet-probability Probability P that tracks associated with a jet come from primary vertex

Heavy flavors: P tends to 0.

b-tagging at CDF

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Signal and Background Modeling

Signal modeling Pythia (or Herwig) do not reproduce correct NLO pt distribution for the b jets in the t-channel Use MadEvent generator

generate b+q t+q’ and g+q t+b+q’ separately and merge them to reproduce the b pt spectrum from NLO calculations (ZTOP)

Background modelingttbar and non-top background Top pairs: Pythia, (tt) = 6.7 pb. Non-top:

W + heavy flavor (62%): Alpgen mistags (25%) and non-W (10%): estimated from data diboson (WW, WZ, ZZ) production (3%): Pythia

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Signal and Background Modeling

Expected number of signal and background events compared with observation for L = 162 pb-1

Combined and 1-tag signal mostly dominated by t-channel (65-70%) 2-tags signal: only s-channel Total background is dominated by non-top events (89%) Observations are in good agreement with predictions

Published CDF results: Phys. Rev. D 71, 012005 (2005)

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Separate Search for Single Top

Maximum likelihood technique to extract the signal content from the dataPerform search to separate s-channel and t-channel events: use variables that help discriminate the two channels.

For t-channel use kinematical boost In proton-antiproton collisions:

top production: light quark jet goes in p direction anti-top: light quark jet goes in p-bar direction

Correlation between pseudorapidity of untagged jet lepton charge Q

Q. distribution asymmetric for t-channel

s-channel Count double b-tags

g

u

d

b

W+t

z+ direction

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Q. Distributions for Separate Search

Data and stacked MC templates weighted by the expected number of events in the 1-tag sample.

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Likelihood Function

Joint likelihood function for Q. distribution in the 1-tag sample and number of events in 2-tag samples:

Poisson

bg gauss. constraints Syst. Uncert.

4 processes: t-channel (j=1), s-channel (j=2), ttpair and non top (j=3, 4) k: mean number of events in bin k of Q. distribution d: mean number of events in the 2-tags sample nk, nd: observed number in data the background is allowed to float but is constrained to SM predictions 7 sources of systematic uncertainties

t or s-channelk: bin indexj: process index

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Systematic Uncertainties

Systematic acceptance uncertainties for t-channel, s-channel and combined signal

Main systematics:

• b-tagging (7%)• luminosity (6%)• top mass (4%)• JES (4%)

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Expected Sensitivity for Separate Search

Perform N MC experiments to estimate expected sensitivity: For each exp. we integrate out all nuisance parameters (all variables except sig) from Lsig, and calculate the upper limit at 95% C.L. Calculate the median of N individual upper limits

t-channel s-channel

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Posterior probability densities for datat-channel s-channel

The maxima of the probability densities give the most probables values for the cross sections:

Since all these results are compatible with zero, we set upper-limits (at 95% CL):

t-ch = 0.0+2.4-0.0 pb

s-ch = 4.6 ± 3.8 pb

t-ch < 10.1 pbs-ch < 13.6 pb

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Combined Search for Single Top

Measurement of the combined t-channel plus s-channel signal in the data use kinematic variable that discriminates s+t channels from the background total transverse energy: HT distribution (scalar sum of MET, Et of lepton and jets)

same HT distribution for s and t-channels different distributions for ttbar and non-top background

Make a likelihood function similar to that in separate search

Use smoothed HT distributions

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HT Distributions for Combined Search

Data Distributions versus SM Expectation for combined sample

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Limits for Combined Search

Expected sensitivity Probability density from data

s+t (MPV) = 7.7 +5.1-4.9 pb

s+t (95% CL) < 17.8 pbs+t (a-priori) < 13.6 pb at 95% CL

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Summary

No significant evidence for single top quark production set first limits on single top cross section compared with Run I improvement of upper limits of 28% (t-channel) and 20% (s-channel)

Technical improvements of the analysis improved MC modeling for single top events full Bayesian treatment of systematic uncertainties

95% CL limits and most probable x-sections values (pb), with 162 ± 10 pb-1

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Multivariate Analysis

New analysis improvements Resolution of Mlb :

Factors contributing to poor resolution: Jet energy, b-jet choosing, neutrino choosing, lepton energy.

use a kinematic fitter to find the neutrino solution most consistent with measured values of the b-jet and MET

Multivariate likelihoods use a combination of variables to optimize signal and background separation variables include Q*, dijet invariant mass, cos lepton – other jet, HT, ET

likelihood function computed from distributions of each variable compute sensitivities (i.e how much luminosity for discovery ?)

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Conclusion

CDF accumulated more events than in entire Run ISingle top search at the Tevatron is more challenging than anticipated

CDF prepares advanced analysis techniques multivariate (neural-network) analysis

CDF expects 3 signal evidence with 1.5 fb-1

Many improvement are needed to observe single top in the next few years but it is feasible in Run II !

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Backup slides

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